In the automated packaging industry there is required the use of films with coefficient of friction (CoF) values lower than 0.35. For this purpose, there are added to the polyolefins, among other additives, slip agents. The purpose of this class of additives is to facilitate the slipping of the film over its own surface and over the metallic surfaces of the extruders and packaging lines, thereby contributing to an enhancement in productivity.
The slip agents used in the majority of commercially available polyethylene and polypropylene resins are unsaturated primary amides of the types oleamide (C18) and erucamide (C22), which present the advantages of relatively low cost and effectiveness in the sense of promoting the reduction of the CoF values to ≦0.35, a range that is required for use in the automated packaging industry. However, with the use of these additives it is quite difficult to predict and/or reproduce the CoF values in films and laminates due to innumerous factors such as low thermal stability and interaction with other additives present in the polymer, adhesives and printing inks.
It will be worth pointing out that in addition to the amides that belong in the class of migratory slip agents, there is also the use of silicones of high molecular weight, that are classified as non-migratory slip agents. Comparatively to the amides, the silicones are noteworthy for their greater thermal stability, their capacity for immediate reduction of the CoF values after extrusion, but on the other hand they are less effective in achieving low CoF values and may entail organoleptic restrictions.
In the market of flexible films used in the automated packaging industry there is a significant need to develop a formulation that may allow the manufacture of films with low CoF values, and particularly that such formulation be able to maintain constant the values of this property. Currently, even when there are actually achieved CoF values below 0.35, there occurs an increase in those values following the processes of lamination, transportation and storage, and when this occurs, it becomes necessary to apply over the surface of the film a solution rich in slip agents in order to reduce the CoF values. However, the same is not always effective, and when this occurs the use of the film is rendered unfeasible.
During the production of the film, as well as after the processing thereof, there usually occurs an increase in the CoF values, which often reach values ≧0.35, which is inappropriate for use on the automated packaging industry due to the adhesion dragging of the film on the metallic surfaces. What would be ideal would be that, after a period of 24 hours following the extrusion of the film, the CoF values would amount to ≦0.35 and managed to stay constant within that range.
However, there are innumerous factors that contribute to the increase in the CoF values. During the extrusion, the increase in the CoF values occurs due to the loss/degradation of the slip additive due to the low thermal stability thereof at the temperatures used for obtaining the film.
During printing there also commonly occurs an increase in the CoF values due to the interaction between the slip additives and the pigments and/or solvents present in the printing inks. The corona treatment, performed on the surface of the film to allow the adhesion of the printing ink, also contributes to the increase of the CoF values and this occurs due to the low stability of the slip additives relatively to the electric discharge. It is pointed out that in adhesively laminated films, in addition to these interferences, there is one further strong aggravating factor that is the interaction between the slip additives and the lamination adhesive, both having polar characteristics. Finally, the increase in the CoF values occurs after the production of the film due to the high temperatures reached during transportation, storage and use in the automated packaging industry, this being caused by the increase in mobility of the amide, and its consequent preferential migration to the adhesive layer due to its greater chemical affinity therewith.
In spite of the critical aspect of the application and the volume of resin used by the industry, there is noted the existence of very few studies aiming to deal with this fault in performance. In this regard, the most recent studies are focused on the use of silicones of high molecular weight, whose restrictions were mentioned above herein. In a very reduced scale there are developments involving the use of amides with chemical structures that are alternative to those of the primary amides, and it will be worth pointing out that in most cases the same are aimed at the obtainment of properties that are alternative with regard to the reduction of the CoF values.
U.S. Pat. No. 3,266,924 relates to a usual application which consists in the joint use of anti-blocking additives and slip agents, more specifically primary amides of fatty acids, for the purpose of easing the slip of the film and minimizing blocking. The blocking occurs when two faces of very thin plastic films come into contact with one another, presenting a certain degree of resistance to separation. The anti-blocking additives act by way of a physical process, reducing the area of contact between the surfaces, and therefore the adherence between the films. It will be worth pointing out that the solution proposed in U.S. Pat. No. 3,266,924 is part of the prior art and presents as a disadvantage the increase of CoF, particularly after processes of printing and lamination of the film.
U.S. Pat. No. 3,647,738 comprehends a polymer composition characterized by providing films with low friction coefficient, low blocking and without occurrence of migration of the slip agent to the surface by means of the incorporation of 0.01-5.0% by weight of a secondary amide of generic formula R—NH—C═O—R′—OH, wherein R is an alkyl radical with 18 to about 22 carbon atoms and R′ is a divalent hydrocarbon radical containing 3-15 carbon atoms. The amides cited in this patent are not usually cited in references in the literature, nor are they used in the industry. Since there is referred the isolated use of secondary erucamide, the proposed solution presents as a limitation a low performance in terms of reduction of the CoF values, particularly in the cases in which this reduction should be achieved at mot 48 hours after the extrusion of the film.
U.S. Pat. No. 5,498,652 discusses the use of amides, more specifically of the omega-acetoxy type of generic formula CH3—COO—(CH2)7CONHR3 for use in polyolefins. In this case, the document describes the improved performance relative to the slip capacity of the film (reduction of the coefficient of friction), and also the promotion of better adhesion of printing inks, particularly water-based printing inks. This solution, as in the preceding case, is not usually cited in the literature nor used in the industry. As it is a case of isolated use of secondary erucamide, most probably the proposed solution suffers from the limitation of a low performance in terms of reduction of the CoF values, particularly in the cases in which this reduction should be achieved at lost 48 hours after extrusion of the film.
U.S. Pat. No. 6,497,965 B1 describes the use of bi amides on the outer layer of polypropylene (PP) films), which printing process is realized by rotogravure. As prior art for PP resins there is cited the use of erucamide and behenamide on the outer layer of films printed by rotogravure, and there is presented as constituting inventive step the use of bis amide, more specifically ethylene bis amide (EBO) in this layer. The use of EBO on the external surface is intended to minimize the problems usually observed in processes of impression by rotogravure and the better performance of the EBO is attributed to the absence of deposition thereof on the “doctor blade”. In this manner there is obtained a better printing quality and a lesser number of interruptions for shutdown and cleaning the printing equipment.
Still considering U.S. Pat. No. 6,497,965 B1, it is important to point out that, according to that document, PP films containing the primary amides erucamide and behenamide, which are cited in the prior art, evidence CoF values between 0.15 and 0.35, while the films additivated with EBO evidence CoF values between 0.15 and 0.45. The broader range of CoF for the firm additivated with EBO is coherent with the fact that this additive evidences a molecular weight practically twice as great as that of the primary amides, however this increase in the CoF values is deleterious to the automated packaging industry.
U.S. Pat. No. 7,267,862 B1 discusses the joint use of primary and secondary amides in low-thickness coextruded films, which may or not be laminated with another film of polyolefin, polyamide or polyester. The inventive step is pointed out to consist in the specific use of slip agents in determined proportions and levels. In the field of the claims, the application is extended to mono-extruded or coextruded, either or not laminated, there being pointed out a better performance when one of the layers contains a 1:1 blend of the secondary amides oleyl palmitamide and stearyl erucamide, additionally in the presence of 25 to 5000 ppm of a primary amide of generic formula R—CONH2. However, the application of this patent extends to films containing 25 to 5000 ppm of primary amide of generic formula R—CONH2 and, at least, one secondary amide of the type R—CO—NH—R′. It will be worth pointing out that among the eight examples of films cited in this patent only in three of them there is set forth the comparative analysis of the CoF values obtained for films additivated with erucamide. Furthermore, the formulation in development presents, in relation to the pattern additivated with erucamide, higher CoF values, which is undesirable in the majority of applications in the automated packaging industry.
Still regarding U.S. Pat. No. 7,267,862 B1 it will be worth pointing out that the additives oleyl palmitamide and stearyl erucamide have a regulatory limitation due to the fact that they are not contemplated in ANVISA1 Resolution No. 105, dated May 19, 1999, and RDC 17/08, which establishes the general provisions for packages and equipment made of plastic material in contact with foodstuffs. In Brazil this renders unfeasible the use of formulations indicated in U.S. Pat. No. 7,267,862 B1 for use in packages that come into contact with foodstuffs, and this is one of the main applications in the industry of flexible films. 1 Translator's note: ANVISA is the acronym for the Brazilian Health Supervision Agency [Agência Nacional de Vigilância Sanitária]
Based on this search of the literature realized in a patent database it is noted that the prior art contemplates the joint use of primary and secondary amides having specific structures. However, the existing studies are not related to the reduction of the CoF values (U.S. Pat. No. 6,497,965 B1) or present some limitations in the sense of achieving low CoF values and/or present limitations in terms of approval for use in contact with foodstuffs (U.S. Pat. No. 7,267,862 B1).
On analyzing the performance of the amides in the reduction of the CoF values it is necessary to take into account the molecular weight, the presence of unsaturated bonds and the structure itself: primary amide, secondary amide or secondary amide of the bis amide type.
By means of a comparative analysis between the performances of the amides in the reduction of the CoF values, it is noted that the unsaturated amides (oleamide and erucamide) have a superior performance with relation to the corresponding saturated amides (behenamide and stearamide). On the other hand, the saturated amides evidence a better performance in blocking reduction and greater thermal stability.
The presence of the unsaturated bond impacts the migration ability, and mostly, the form in which the same crystallizes on the surface of the film. Comparative studies between the erucamide and the behenamide with the use of AFM (Atomic Force Microscopy or Scanning Force Microscopy) show differences between the crystalline structures formed on the surface of the film. The better efficiency of the erucamide in reducing the CoF values is related to the fact that its crystalline structure is larger, flatter and less rigid, favoring the slipping thereon and therefore reducing the CoF values.
The molecular weight of the amide impacts the migration velocity thereof and its thermal stability. The greater the molecular weight, the slower will be the migration velocity of the amide, and consequently the lesser will be the velocity and capacity of reduction of the CoF values.
As previously cited, the isolated use of the secondary amides is not effective for reducing the CoF values such as to fulfill the needs of the automated packaging industry.
In turn, the secondary amides of the bis amides type have as their main application the use in Ethylene-vinyl acetate resins as an “anti-tack” [anti-grip] agent. This application is in the public domain, and in such connection there may be cited as an example the patent RE. 32.325—Tack Free Polymer Pellets.
In this connection, in order to facilitate the slip of the films in the processes of printing, lamination and automatic packaging there are basically used in the prior art primary amides of the types erucamide, oleamide and behenamide, and secondary amides of the types oleyl palmitamide and stearyl erucamide.
The available solutions present limitations in the sense of generating flexible films with low CoF values (lower than 0.35) and, particularly, of generating films capable of maintaining the CoF values under control after the lamination process and after exposure to critical conditions (time and temperature) of transportation and/or storage in the automated packaging industry. This invention is intended to fulfill this problem of performance.